Medical devices designed to be implanted in a patient's body are typically operated by means of electrical power. Such medical devices include electrical and mechanical stimulators, motors, pumps, etc, which are designed to support or stimulate various body functions. Electrical power can be supplied to such an implanted medical device from a likewise implanted battery or, more preferably, from an external energy source that can supply any needed amount of electrical power more or less continuously without requiring repeated surgical operations.
An external energy source may be placed just beneath the patient's skin in the vicinity of the implanted device. So-called TET (Transcutaneous Energy Transfer) devices are known that can transfer wireless energy to an internal energy receiver which is located inside the patient and connected to the medical device for supplying received energy thereto. Thereby, no leads or the like penetrating the skin need to be used for connecting the medical device to an external energy source, such as a battery.
A TET device typically comprises an external energy source including a primary coil adapted to inductively transfer any amount of wireless energy, by inducing voltage in a secondary coil of an internal energy receiver which is implanted preferably just beneath the skin of a patient. The highest transfer efficiency is obtained when the primary coil is positioned close to the skin adjacent to and in alignment with the secondary coil, i.e. when a symmetry axis of the primary coil is parallel to that of the secondary coil.
Typically, the amount of energy required to operate an implanted medical device may vary over time depending on the operational characteristics of the device. For example, the device may be designed to switch on and off at certain intervals, or otherwise change its behaviour, in order to provide a suitable electrical or mechanical stimulation, or the like. Such operational variations will naturally result in corresponding variations with respect to the amount of required energy.
Furthermore, the position of the primary coil relative to the implanted secondary coil is a factor that affects the efficiency of the energy transfer, which highly depends on the distance between the coils and the relative angle between the coils. Initially, during set-up of a TET-system, it may be difficult to find an optimal position of the primary coil, and to maintain it in that position. During operation of the medical device, the patient's movements will typically change the relative spacing of the two coils such that the transfer efficiency greatly varies. Hence, changes in coil spacing result in a corresponding variation of the induced voltage.
If the transfer efficiency becomes low, the amount of energy supplied to the medical device may be insufficient for operating the device, so that its action must be momentarily stopped, naturally disturbing the intended operation of the device. Although this situation is more likely to occur when large amounts of power are to be transferred for supplying sufficient energy to an implant, it can be regarded as a general problem.
On the other hand, the energy supplied to the medical device may also increase drastically, if the relative positions of the coils change in a way that unintentionally increases the transfer efficiency. This situation can cause severe problems since the implant cannot “consume” the suddenly very high amount of supplied energy. Unused excessive energy must be absorbed in some way, resulting in the generation of heat, which is highly undesirable. Hence, if excessive energy is transferred from the primary coil to the secondary coil, the temperature of the implant will increase, which may damage the surrounding tissue or otherwise have a negative effect on body functions. It is therefore highly desirable to always supply the right amount of energy to an implanted medical device during operation.
Methods are known for controlling the amount of transferred energy in response to measured conditions in the receiving implant. For example, U.S. Pat. No. 5,995,874 discloses a TET system in which the amount of transmitted energy from a primary coil is controlled in response to an indication of measured characteristics of a secondary coil, such as load current and voltage. The transmitted energy can be controlled by varying the current and voltage in the primary coil, transmission frequency or coil dimensions. In particular, a change is effected in the saturation point of the magnetic field between the coils, in order to adjust the power transfer efficiency. However, it is not likely that this solution works well in practice, since a saturation point in the human tissue would hardly occur, given the magnetic field levels that are possible to use. Moreover, if the energy transmission must be increased considerably, e.g. to compensate for losses due to variations in alignment and/or spacing between the coils, the relatively high radiation generated may be damaging or unhealthy or unpleasant to the patient, as is well known.
An effective solution is needed for accurately controlling the amount of transferred energy to an implanted medical device, where the correct amount of energy is always supplied to the device for proper operation thereof. In particular, excessive energy transfer resulting in raised temperature at the medical device, and/or power surges should be avoided. Furthermore, the transmitted electromagnetic energy should be kept at a minimum, in order to avoid tissue damages and other unhealthy or unpleasant consequences for the patient.